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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

130

SUPERPHOSPHATES
SUPERPHOSPHATES

In these products the phosphoric acid occurs mostly us mono calcium
phosphate, CatHaPO^a, and to a less extent as dicalcium phosphate,
CaHP04, that is, in two forms soluble in water and in ammonium citrate ;
smaller quantities may occur as tncaldum phosphate, insoluble, in water
or the citrate, but soluble in acids. Superphosphates contain also free
phosphoric and sulphuric acids, calcium sulphate, silica, and other extraneous
substances, according to their origin (see later).

Superphosphates are distinguished as bone-   superphosphates ;   mineral
superphosphates ;   and double, triple, and enriched, super phosphates, which
are obtained by treating the phosphates with phosphoric acid.
The analysis of these products includes the following :

1.  Moisture.Sec- General Methods, 2.

2,  Phosphoric Anhydride.-  That soluble in water and ammonium
citrate, and that soluble in water alone, are. determined.

A, PHOSPHORIC ANHYDKIDK sor.UHi.i-; IN WATKK AND IN AMMONIUM
CITRATE (Appiani's method}. This requires the solutions (ammonium
citrate, magnesia mixture1 and ammonia) prescribed lor the determination
of the total phosphoric acid (see General Methods, 4) : 5 grams of the super-
phosphate (2-5 grams with double or triple superphosphate) are made into
a paste; in a mortar with 4.0 50 c,c. of water, allowed to si ami a few minutes
and the liquid decanted on to a pleated filter, the filtrate being collected in 
a 250 e,e. measuring lla.sk. This treatment with water is repeated three or
four times, the operations being regulated so 11 nil the digestion lasts only a
few minutes and the. filter is always empty when the decanted liquid is placed

in it.    The whole of the solid matter is .finally washed on to the filter and

there washed unlil the volume of the. total nitrate occupies nearly 250 c.e.. ;

a few drops of hydrochloric, or nitric acid are then added and Ilie volume,

made up with water (aqueous solution],

The alter and its contents an- introduced into another 250 c.c. measuring

flask and digested with 100 c.c. of the ammonium citrate solution l for an

hour a,t 35 40", with frequent   shaking; when  cool, Hie  volume is rnatU;

up to  250  c.c.  with   water   and   the  liquid  shaken  and   filtered  (citric

solution).

To 50 c.c, of the aqueous solution are added 50 c.c.. of the citric solution,
50 c.c. of water, 50 c.c. of ammonia, (1) o-tju) and then, gradually and with
shaking, 50 c.c. of magnesia, mixture. Alter the whole has been well shaken,

1 This amount of citrate is usually more than lutllu tent lu dissolve all Uu- dkaldum
phosphate in tin- residue insoluble in water from ordinary < ummercial superphosphates,
inwhii'li form I'our-lilihs in nine ten 1 ItsoI' Ihr phosphottc at id soluble in citrate, is soluble
in water. With precipitated phosphates, with superphosphates of high Brittle, but
poor in phosphoric acid soluble in water, and with phosphates quite free from mommal-
dura phosphate, so that part of the phosphate init;hl remain nndi'<solved, it; is advisable
to nse more citrate or to work with a .smaller quantity ol ''Ubstauee,

With double and triple superphosphates, meat mianu, or excessively dry super-
phosphates, in order to include ;I]SM any pvro- and m'ta phuHphnrie acids present it is
twlvisiible, before precipitating the jihoivphorir ucid. t> he,1,1 the slut.um for some time
with a little nitric, add to tranhfonn into phosphoric at id the pyrti ami meta-pho.Hplwric
acids wliich may be fonwul during drying.